This invention is generally directed to the field of switching systems, and more particularly relates to an improved method of call control in a telephone switching system and to an improved telephone switching system.
There has been rapid developments in telephone switching systems in recent years. Many such conventional telephone switching systems operate to selectively connect not only telephone sets but also data terminals to a plurality of other telephone sets and data terminals. Generally, the exchange for the switching system begins a call sequence by selectively connecting a calling terminal (telephone set or data terminal) to a called terminal as designated by the calling terminal. The called terminals respond accordingly to the exchange to complete the connection. The exchange also disconnects the connection in accordance with the request of either terminal to end or finish the call. Therefore, each terminal is required to convey several types of commands to the exchange. Typically, when the user, e.g., picks up a telephone handset, dials a telephone number to make a call and then hangs up, the call request, the designation of the called party and the call completion are all conveyed to the exchange. Also, when the called party picks up the phone to answer the call, an answer response is conveyed to the exchange.
The above call procedures are also followed when data terminals are used, although some communications are conveyed to the exchange automatically while others are in response to manual operation by the user. To convey the various commands and other communications between data terminals and the exchange, a data interface unit is interposed between the exchange and the terminal. The interface unit is sometimes called a data service unit, a data control unit, a network control unit, or the like. In the present specification, the interface unit will be referred to as a data interface unit or DIU. Regardless of the nomenclature, the DIU serves at least the minimum purpose of conveying call request and response information to the exchange.
In more cases, each user has his own telephone set and in some cases the users has his own data terminal as well. Thus, a telephone set and a DIU for the data terminal are often located in the same place, e.g., on a desk. Therefore, the user has to provide space not only for the telephone set but also for the data interface unit and the data terminal. The space occupied by the data interface unit can be a relatively large space because it usually has several keys and a dial pad, for instance, for its operation. In addition, the user is required to operate both the telephone set and the data interface unit. Moreover, data interface units are relatively expensive, resulting in an overall increase in the cost of the entire system.